A large proportion of chlorine and caustic produced throughout the world is manufactured in diaphragm type electrolytic cells wherein the opposed anode and cathode are separated by a fluid permeable diaphragm which may be of asbestos, a polymer film or a polymer film supported on asbestos. The diaphragm defines separate anolyte and catholyte compartments. Chlorine is produced in the former, aqueous sodium hydroxide in the latter.
In operation, brine, preferably saturated, is fed to the anolyte compartment. The brine passes through the diaphragm into the catholyte compartment where an aqueous sodium hydroxide solution of about 11 to 18 percent is produced. This solution is contaminated by sodium chloride which must be separated. After separation, the caustic solution is concentrated to produce the commercial product.
The permionic exchange membrane type electrolytic cell has been utilized as an improvement over the diaphragm type. The permionic membrane replaces the diaphragm, and is characteristically different from it since, in contrast to the diaphragm, it is substantially impervious to water and to sodium chloride. The exchange membrane selected for the production of chlorine and caustic is usually a cation exchange membrane which permits the passage of sodium ions into the catholyte, but prevents back-migration of OH ions into the anolyte. As a result, relatively pure caustic substantially free of sodium chloride is produced in the catholyte, and high grade chlorine is produced at the anode.
A number of cation exchange membranes are known.
U.S. Pat. Nos. 3,887,499 and 3,657,104 describe permselective cation exchange membranes comprising a hydrocarbon polymer backbone with pendant carboxylic and sulfonic groups.
U.S. Pat. No. 3,878,072 describes cation exchange membranes which are hydrolyzed copolymers of a perfluorinated hydrocarbon and either a fluorosulfonated perfluorovinyl ether or a sulfostyrenated perfluorinated ethylene propylene polymer. In either event, the characteristic feature of the membrane is the sulfonyl group as the only functional group.
U.S. Pat. No. 3,853,721 which issued on Dec. 10, 1974 describes asbestos diaphragms containing from about 0.01 to 22 weight percent, based on the weight of the diaphragm, of an ion exchange resin which is a fluorocarbon polymer characterized by the presence of the following groups:
sulfonic--SO.sub.3 H PA1 fluoromethylene sulfonic--CF.sub.2 SO.sub.3 H PA1 benzene sulfonic--.phi.SO.sub.3 H PA1 chloromethylene sulfonic--CCl.sub.2 SO.sub.3 H PA1 carboxylic--COOH PA1 phosphoric--PO.sub.3 H.sub.2 PA1 phosphorous--PO.sub.2 H.sub.2 PA1 phenolic--.phi.OH PA1 1. Decreased back-migration of hydroxyl ions. PA1 2. Increased current efficiency at high current density even when the concentration of sodium hydroxide in the catholyte is high. PA1 3. Increased purity of the sodium hydroxide solution produced in the cathode because of the resistance of the membrane to permeation by sodium chloride. PA1 4. Increased purity of chlorine produced at the anode. PA1 5. Resistance to oxidation. PA1 1. Their useful life in operation is surprisingly long. PA1 2. Power consumption in units in which they are employed is surprisingly low.
A characteristic feature of these diaphragms is that they are gas and electrolyte permeable in contrast to permionic membranes, such as the cationic ion exchange membranes of this invention which are substantially impermeable to electrolytes, but permit the passage of ions. Another feature, according to the patent, is that they have a resistance voltage drop across the diaphragm of as much as 0.2 to 0.3 volt less than an untreated asbestos diaphragm of the same thickness.
A problem with the use of diaphragm electrolysis, as pointed out above, is that the permeability of the diaphragm to sodium chloride results in accumulation of this material in the catholyte. This concentration may be as high as 17%. When attempts are made to produce concentrated solutions of sodium hydroxide in the catholyte by evaporation of water, it is necessary to use an expensive apparatus as well as a large amount of energy.
U.S. Pat. No. 3,301,893 discloses certain fluorocarbon ethers containing both carboxyl and sulfonyl groups. The products of this patent, however, are of such low molecular weight that they cannot be utilized for the preparation of cation exchange membranes.